A conventionally employed lubricating oil composition for lubricating a trunk piston marine diesel engine contains a variety of additive components which serve to improve various performances of the lubricating oil and prolong life of the lubricating oil.
A typical lubricating oil for the trunk piston diesel engine comprises an overbased metal-containing detergent such as an overbased calcium sulfonate (overbased alkylbenzenesulfonic acid calcium salt detergent), an overbased calcium phenate (overbased sulfurized alkylphenol calcium salt detergent) or an overbased calcium salicylate (overbased alkylsalicylic acid calcium salt detergent) and additives such as an ashless dispersant dispersed or dissolved in a base oil having a kinematic viscosity of approx. 22-300 mm2/s (measured at 40° C.).
Thus, the lubricating oil for the trunk piston diesel engine contains an overbased metal-containing detergent for neutralizing acidic sulfur oxides (particularly, sulfuric acid) which are produced after combustion of sulfur components contained in petroleum fuels such as A heavy oil or C heavy oil which contains a large amount of sulfur components. The metal-containing detergent serves in combination with an ashless dispersant to neutralise the sulfur oxides and further to uniformly disperse combustion residues such as soot and sludge produced by combustion of the fuel and deterioration of the lubricating oil, whereby keeping the combustion residues from depositing on various parts of the combustion engines such as a piston, piston grooves and a cylinder liner surface.
Recently, studies for the purpose of decreasing fuel consumption in a trunk piston diesel engine have been performed. It is known that the fuel consumption is affected by wear of a top ring of a piston. In more detail, when excessive wear occurs on the piston ring, a fuel gas is apt to escape from the space between the cylinder liner and the piston ring suffering wear. Therefore, the fuel consumption increases.
Recently, lubricating oils formulated specifically for trunk piston diesel engines generally utilize an overbased sulfurized alkylphenol calcium salt detergent and an overbased alkylsalicylic acid calcium salt detergent as the main metal-containing detergent, so as to increase detergency of the internal combustion engines subjected to operations at high temperatures.
Japanese Patent Provisional Publication 2005-263860 (Patent Publication 1) teaches that a lubricating oil composition containing a combination ox a salicylate detergent having one hydrocarbyl group containing 8 to 19 carbon atoms and a salicylate detergent having one hydrocarbyl group having 20 to 32 carbon atoms is favorably employed for lubricating internal combustion engines for the use in marine vehicles, power generation and railways, particularly, a four-stroke trunk piston internal combustion engine.
Japanese Patent Provisional publication 2005-263861 (Patent Publication 2) teaches that a lubricating oil composition containing a combination of (A) an alkali metal salicylate or an alkaline earth metal salicylate and/or its basic or overbased salt and (B) a metal-containing detergent having a BN of 240 mg·KOH/g or more and/or a metal ratio of 4.5 or more is favorably employed for lubricating internal combustion engines for the use in marine vehicles, power generation and railways, particularly, a four-stroke trunk piston internal combustion engine.